arXiv subjects

Chengwang Niu

Publications and source records attributed to Chengwang Niu.

At least 19 recordsLinked to original sources

arXiv2026

Higher-Order Topological Phase in the Two-Dimensional Type-IV Magnet MgCr$_2$O$_4$

Type-IV two-dimensional (2D) magnetism-a newly classified collinear magnetic phase featuring nonrelativistic spin degeneracy and spin-orbit-coupling-induced momentum-dependent spin splitting-extends the symmetry classification of collinear magnets, opening new opportunities for unconventional topological quantum states. Here, we reveal that the recently proposed two-dimensional type-IV 2D magnet MgCr$_2$O$_4$ hosts an intrinsic higher-order topological insulating phase, featuring $\mathcal{C}_{3z}$-protected corner states and a nontrivial rotational topological invariant of $\chi^{(3)}$ = $\{-2,4\}$ with a quantized fractional corner charge of $4e/3$. Spin-orbit coupling breaks the spin-degeneracy-enforcing symmetry $[C_{2}||M_z]$ while preserving the crystalline $\mathcal{C}_{3z}$ rotational symmetry that protects the higher-order topological phase, thereby enabling spin splitting to coexist with the nontrivial topology. Furthermore, the higher-order topological phase remains intact throughout a wide range of biaxial strains without band-gap closing and topological phase transition, demonstrating the robustness of the symmetry-protected topological state against external perturbations. Our work establishes a direct connection between type-IV magnetic system and higher-order topology, providing a new route for symmetry-engineered magnetic topological quantum states.

physics.comp-ph↗

arXiv2026

Floquet-Engineered Odd-Parity Altermagnetic Higher-Order Topology in a Two-Dimensional Antiferromagnet Cr$_2$CH$_2$

Periodic driving provides a platform to dynamically tailor quantum states of matter, yet its impact on symmetry-protected topological phases remains incompletely understood. Here, we demonstrate that periodic driving enables the realization of an odd-parity altermagnetic (AM) higher-order topological insulator (HOTI) phase in the Cr$_2$CH$_2$ monolayer. In equilibrium, Cr$_2$CH$_2$ is a 2D antiferromagnetic (AFM) HOTI protected by $\mathcal C_3$ rotational symmetry, characterized by a symmetry indicator $\chi^{(3)}$ = $\{-2,1\}$ and robust corner states. Under circularly polarized light (CPL), the system develops a f-wave altermagnetic state governed by the symmetry $[C_{2}||\overline{3}_{001}]$ with odd-parity spin splitting. Despite substantial Floquet-induced band renormalization, the $\mathcal C_3$-protected corner states remain intact over a broad range of driving strengths, highlighting the altermagnetic higher-order topology under Floquet driving. As the light intensity increases, the system gradually evolves into an altermagnetic semimetallic state. These results establish a direct connection between magnetism and topology in a periodically driven AFM system, offering a route toward the control of coupled spin and topological transport.

physics.comp-ph↗

arXiv2026

Second-order topology in two-dimensional azulenoid kekulene carbon lattices

The discovery of higher-order topological insulator (HOTI) has established a new paradigm for understanding symmetry-constrained boundary electronic states. Here, based on first-principles calculations, we demonstrate the emergence of HOTI phase in organic lattices of two-dimensional azulenoid-kekulene-type carbon allotropes, namely AKC-[3,3] and AKC-[6,0]. Enabled by the $C_6$ rotational symmetry, the nontrivial bulk topology is confirmed through the topological invariant and fractionally quantized corner charge, giving $\{[M^{(I)}_{2}],[K^{(3)}_{2}]\}$ = $\{0,2\}$ and $Q_{\mathrm{corner}} = e/3$, respectively, accompanied by the emergence of exotic corner states in nanoflakes. Notably, the structural modifications are explored, revealing that in the derived structure PAK-[6,0], whose corner-localized states are preserved, highlighting the robustness of the higher-order topological phase. These findings highlight azulenoid-kekulene-based carbon allotropes as a promising platform to explore the interplay between structural design, crystalline symmetry, and higher-order topological boundary responses in two dimensional carbon systems.

cond-mat.mtrl-sci↗

arXiv2026

Magnonic Quantum Spin Hall Effect with Chiral Magnon Transport in Bilayer Altermagnets

Altermagnetism has attracted considerable interest, yet its associated spintronic phenomena have so far been largely confined to electronic systems. In this work, we uncover a universal symmetry-based strategy for realizing topological altermagnets with the magnonic quantum spin Hall effect, as evidenced by a nonzero spin Chern number and protected helical edge states. Moreover, we demonstrate that chiral magnon splitting in altermagnets gives rise to an intrinsically anisotropic, momentum-resolved thermal Hall response, sharply contrasting with those in ferromagnets and antiferromagnets, thus offering enhanced flexibility for selective manipulation. As a concrete material realization, first-principles calculations and Heisenberg-DM model analysis reveal that V$_2$WS$_4$ bilayer exhibits $d$-wave altermagnetism, integer spin Chern number with helical magnon edge states, and the nonzero momentum-locked thermal Hall conductivity. Our results establish a direct link between topological magnons and altermagnetism, opening new avenues for dissipationless magnonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Coexistence and tunability of orbital and spin Hall effects in RuO$_2$

Altermagnetic materials, especially RuO$_2$, have recently attracted considerable attention for their unique magnetic properties and energy-efficient spintronic applications. However, recent experimental studies have reported highly conflicting signatures regarding altermagnetic spin splitting and charge--spin interconversion (CSI) in RuO$_2$. While some experiments link efficient CSI to non-relativistic altermagnetic spin-splitting effects, others observe large CSI signals in non-spin-splitting RuO$_2$, which are instead explained by relativistic inverse spin Hall effects. In this work, based on first-principles calculations, we reveal that these controversial experimental results originate from a phase-dependent coexistence and relative dominance of the orbital Hall effect (OHE) and spin Hall effect (SHE) in RuO$_2$. We systematically investigate the OHE and SHE in both altermagnetic and nonmagnetic phases of RuO$_2$. Our results show that the altermagnetic state hosts a giant OHE that exceeds the SHE by two orders of magnitude and carries an opposite sign. This dominant OHE can generate experimentally observed "SHE-like" voltages through orbital-to-spin conversion, explaining previously reported altermagnetic CSI signals. In contrast, OHE of nonmagnetic RuO$_2$ is suppressed and a large relativistic SHE emerges, in agreement with recent angle-resolved photoemission and spin-pumping experiments. Finally, we demonstrate that the coexistence of OHE and SHE is tunable via chemical doping, enabling on-demand modulation of CSI in in RuO$_2$. Our work provides a new physical mechanism for understanding CSI in RuO$_2$ and highlights the central role of orbital transport.

cond-mat.mtrl-sci↗

arXiv2025

Pyrochlore NaYbO2: A potential Quantum Spin Liquid Candidate

The search for quantum spin liquids (QSL) and chemical doping in such materials to explore superconductivity have continuously attracted intense interest. Here, we report the discovery of a potential QSL candidate, pyrochlore-lattice beta-NaYbO2. Colorless and transparent NaYbO2 single crystals, layered alpha-NaYbO2 (~250 um on edge) and octahedral beta-NaYbO2 (~50 um on edge), were grown for the first time. Synchrotron X-ray single crystal diffraction unambiguously determined that the newfound beta-NaYbO2 belongs to the three-dimensional pyrochlore structure characterized by the R-3m space group, corroborated by synchrotron X-ray and neutron powder diffraction and pair distribution function. Magnetic measurements revealed no long-range magnetic order or spin glass behavior down to 0.4 K with a low boundary spin frustration factor of 17.5, suggesting a potential QSL ground state. Under high magnetic fields, the potential QSL state was broken and spins order. Our findings reveal that NaYbO2 is a fertile playground for studying novel quantum states.

cond-mat.str-el↗

arXiv2025

Detection and control of electronic orbital magnetism by spin waves in honeycomb ferromagnets

Exploring and manipulating the orbital degrees of freedom in solids has become a fascinating research topic in modern magnetism. Here, we demonstrate that spin waves can provide a way to control electronic orbital magnetism by the mechanism of scalar spin chirality, allowing for experimental detection using techniques such as the magneto-optical Kerr effect and scanning transmission electron microscopy. By applying linear spin wave theory, we uncover that electronic magnon-driven orbital magnetization is extremely sensitive to the character of the magnonic excitations. Furthermore, we show that both the induced electronic orbital magnetism and the Nernst transport properties of the orbital angular momentum can be regulated by the strength of the Dzyaloshinskii-Moriya interaction, Kitaev interaction, as well as the direction and magnitude of the external magnetic field. We argue that magnon-mediated electronic orbital magnetism presents an emergent variable which has to be taken into account when considering the physics of coupling magnonic excitiations to phonons and light.

cond-mat.str-el↗

arXiv2024

Topology-engineered orbital Hall effect in two-dimensional ferromagnets

Recent advances in manipulation of orbital angular momentum (OAM) within the paradigm of orbitronics present a promising avenue for the design of future electronic devices. In this context, the recently observed orbital Hall effect (OHE) occupies a special place. Here, focusing on both the second-order topological and quantum anomalous Hall insulators in two-dimensional ferromagnets, we demonstrate that topological phase transitions present an efficient and straightforward way to engineer the OHE, where the OAM distribution can be controlled by the nature of the band inversion. Using first-principles calculations, we identify Janus RuBrCl and three septuple layers of MnBi$_2$Te$_4$ as experimentally feasible examples of the proposed mechanism of OHE engineering by topology. With our work we open up new possibilities for innovative applications in topological spintronics and orbitronics.

cond-mat.mtrl-sci↗

arXiv2022

Ferroelectric higher-order topological insulator in two dimensions

The interplay between ferroelectricity and band topology can give rise to a wide range of both fundamental and applied research. Here, we map out the emergence of nontrivial corner states in two-dimensional ferroelectrics, and remarkably demonstrate that ferroelectricity and corner states are coupled together by crystallographic symmetry to realize the electric control of higher-order topology. Implemented by density functional theory, we identify a series of experimentally synthesized two-dimensional ferroelectrics, such as In$_2$Se$_3$, BN bilayers, and SnS, as realistic material candidates for the proposed ferroelectric higher-order topological insulators. Our work not only sheds new light on traditional ferroelectric materials but also opens an avenue to bridge the higher-order topology and ferroelectricity that provides a nonvolatile handle to manipulate the topology in next-generation electronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Terahertz spin-to-charge current conversion in stacks of ferromagnets and the transition-metal dichalcogenide NbSe$_2$

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe$_2$ in ultra-high-vacuum-grown F|NbSe$_2$ thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe$_2$ thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe$_2$. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe$_2$ with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe$_2$ to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe$_2$ for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.

cond-mat.mes-hall↗

arXiv2022

Orbital shift-induced boundary obstructed topological materials with a large energy gap

We propose boundary obstructed topological phases caused by Wannier orbital shift between ordinary atomic sites, which, however, cannot be indicated by symmetry eigenvalues at high symmetry momenta (symmetry indicators) in bulk. On the open boundary, Wannier charge centers can shift to different atoms from those in bulk, leading to in-gap surface states, higher-order hinge states or corner states. To demonstrate such orbital-shift-induced boundary obstructed topological insulators, we predict eight material candidates, all of which were overlooked in present topological databases. Metallic surface states, hinge states, or corner states cover the large bulk energy gap (for example, more than 1 eV in TlGaTe$_2$) at related boundary, which are ready for experimental detection. Additionally, we find these materials are also fragile topological insulators with hourglass like surface states.

cond-mat.mes-hall↗

arXiv2022

Doubled Quantum Spin Hall Effect with High-Spin Chern Number in $\alpha$-Antimonene and $\alpha$-Bismuthene

The discovery of quantum spin Hall effect has ignited the field of topological physics with vast variety of exotic properties. Here, we present the emergence of doubled quantum spin Hall effect in two dimensions characterized with a high spin Chern number of ${\mathcal C_S}=2$ and two pairs of helical edge states. Although is overlooked and invisible in topological quantum chemistry and symmetry indicator theory, the already experimentally synthesized $\alpha$-antimonene and $\alpha$-bismuthene are revealed as realistic material candidates of predicted topological states with band inversions emerging at generic $k$-points, rather than the high-symmetry momenta. Remarkably, the nontrivial energy gap can be as large as 464 meV for $\alpha$-bismuthene, indicating the high possibility of room-temperature observation of the doubled quantum spin Hall effect. Moreover, a four-band effective model is constructed to demonstrate further the feasibility of attaining this type of nontrivial topology. Our results not only uncover a novel topological character of antimony and bismuth, but will also facilitate the experimental characterization of the previously overlooked hidden topology.

cond-mat.mtrl-sci↗

arXiv2021

Hidden wallpaper fermion and third-order topological insulator

Nonsymmorphic symmetry can induce exotic wallpaper fermions, e.g., hourglass fermion, fourfold-degenerate Dirac fermion, and M\"obius fermion, as commonly believed only in nonsymmorphic wallpaper groups. Here, we extend the notion of wallpaper fermions to symmorphic wallpaper groups, and remarkably uncover the emergence of long-awaited third-order topological insulators. The symmetry analysis and k $\cdot$ p models reveal that nonessential symmetries play an essential role for obtaining the previously overlooked hidden surface spectrum. Based on this, we present the hourglass fermion, fourfold-degenerate Dirac fermion, and M\"obius fermion in the (001) surface of Tl$_4$XTe$_3$ (X = Pb/Sn) with a symmorphic wallpaper group $p4m$. Remarkably, 16 helical corner states reside on eight corners in Kramers pair, rendering the first real electronic material of third-order topological insulator. A time-reversal polarized octupole polarization is defined to uncover the nontrivial third-order topology, as is implemented by the 2$^{nd}$ and 3$^{rd}$ order Wilson loop calculations. Our results could considerably broaden the range of wallpaper fermions and lay the foundation for future experimental investigations of third-order topological insulators.

cond-mat.mtrl-sci↗

arXiv2018

Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets

The concepts of Weyl fermions and topological semimetals emerging in three-dimensional momentum space are extensively explored owing to the vast variety of exotic properties that they give rise to. On the other hand, very little is known about semimetallic states emerging in two-dimensional magnetic materials, which present the foundation for both present and future information technology. Here, we demonstrate that including the magnetization direction into the topological analysis allows for a natural classification of topological semimetallic states that manifest in two-dimensional ferromagnets as a result of the interplay between spin-orbit and exchange interactions. We explore the emergence and stability of such mixed topological semimetals in realistic materials, and point out the perspectives of mixed topological states for current-induced orbital magnetism and current-induced domain wall motion. Our findings pave the way to understanding, engineering and utilizing topological semimetallic states in two-dimensional spin-orbit ferromagnets.

cond-mat.mes-hall↗

arXiv2017

Mixed Weyl semimetals and dissipationless magnetization control in insulators by spin-orbit torques

Reliable and energy efficient magnetization switching by electrically-induced spin-orbit torques is of crucial technological relevance for spintronic devices implementing memory and logic functionality. Here we predict that the strength of spin-orbit torques and the related Dzyaloshinskii-Moriya interaction in topologically non-trivial magnetic insulators can exceed by far that of conventional metallic magnets. In analogy to the quantum anomalous Hall effect, we explain this extraordinary response in absence of longitudinal currents as a hallmark of magnetic monopoles in the electronic structure of systems that are interpreted most naturally within the framework of mixed Weyl semimetals. We thereby launch the effect of spin-orbit torque into the field of topology and reveal its crucial role in mediating the topological phase transitions arising due to the complex interplay between magnetization direction and momentum-space topology. The concepts presented here may be exploited to understand and utilize magneto-electric coupling phenomena in insulating ferromagnets and antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2017

Topological Nodal-line Semimetals in Two Dimensions with time-reversal symmetry breaking

Topological nodal-line semimetals (TNLSs) exhibit exotic physical phenomena due to a one-dimensional (1D) band touching line, rather than discrete (Dirac or Weyl) points. While so far proposed two-dimensional (2D) TNLSs possess closed nodal lines (NLs) only when spin-orbit coupling (SOC) is neglected, here using Na$_3$Bi trilayers as an example, we show that 2D TNLSs can been obtained from topological (crystalline) insulators (TI/TCI) by time-reversal symmetry breaking even in the presence of SOC. We further reveal that these obtained NLs are protected by crystalline mirror symmetry, while a mirror symmetry breaking perturbation opens a full gap thus giving rise to a phase transition from 2D TNLS to a quantum anomalous Hall insulator (QAHI). We thereby uncover a close correlation between various topological phases. Remarkably, a strong spin Hall effect, important for transport applications, is predicted in 2D TNLS. Finally, a Na$_2$CrBi trilayer is proposed to realize the 2D TNLS without extrinsic magnetic field. Our work not only proposes a new strategy for realizing 2D TNLSs with truely closed NLs, but also reveals potential applications of TNLS in spintronics.

cond-mat.mtrl-sci↗

arXiv2017

Two-dimensional topological nodal line semimetal in layered $X_2Y$ ($X$ = Ca, Sr, and Ba; $Y$ = As, Sb, and Bi)

In topological semimetals the Dirac points can form zero-dimensional and one-dimensional manifolds, as predicted for Dirac/Weyl semimetals and topological nodal line semimetals, respectively. Here, based on first-principles calculations, we predict a topological nodal line semimetal phase in the two-dimensional compounds $X_2Y$ ($X$=Ca, Sr, and Ba; $Y$=As, Sb, and Bi) in the absence of spin-orbit coupling (SOC) with a band inversion at the M point. The mirror symmetry as well as the electrostatic interaction, that can be engineered via strain, are responsible for the nontrivial phase. In addition, we demonstrate that the exotic edge states can be also obtained without and with SOC although a tiny gap appears at the nodal line for the bulk states when SOC is included.

cond-mat.mtrl-sci↗

arXiv2017

Robust Dual Topological Character with Spin-Valley Polarization in a Monolayer of the Dirac Semimetal Na$_3$Bi

Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g. mirror) symmetry. The insulating phase, called topological crystalline insulator (TCI), has been intensively investigated and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulator, while so far considered 2D TCIs almost exclusively possess a square lattice structure with the mirror Chern number $\mathcal C_{M} =-2$. Here, we predict theoretically that hexagonal monolayer of Dirac semimetal Na$_3$Bi is a 2D TCI with a mirror Chern number $\mathcal C_{M} =-1$. The large nontrivial gap of 0.31 eV is tunable and can be made much larger via strain engineering while the topological phases are robust against strain, indicating a high possibility for room-temperature observation of quantized conductance. In addition, a nonzero spin Chern number $\mathcal C_{S} =-1$ is obtained, indicating the coexistence of 2D topological insulator and 2D TCI, i.e. the dual topological character. Remarkably, a spin-valley polarization is revealed in Na$_3$Bi monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via unusual edge states' behavior under corresponding symmetry breaking.

cond-mat.mtrl-sci↗